Populations and communities

• Populations


  Interacting groups of organisms of the same species living in an area
• Populations


  • Form an important group by interacting with other populations within a community
  • A population is a group of organisms of the same species who live in the same area at the same time able to interbreed with each other
  • If two populations live in different areas, they are unlikely to interbreed with each other
  • If two populations of the same species are geographically isolated for a longer time, they may develop differences in their characters and eventually become different species
• Estimation of population size


  By random sampling
• Estimation of population size by random sampling

  1. Impossible to count all individuals, so count a sample from a small area and multiply by total area

  2. Two common sampling methods for sessile organisms: belt transect, quadrat sampling
• Random quadrat sampling

  • Eliminates bias, suitable for sessile organisms like plants or very small, small moving animals
  • Quadrats randomly placed within a selected area

  1. Generate random numbers to determine distances along and across habitat

  2. Place quadrat at determined distances

  3. Record numbers in each quadrat

  4. Calculate average and multiply by total area to estimate population size
• Capture-mark-release-recapture and the Lincoln index

  • Used to estimate population size for motile organisms
  • Capture, mark, release and then recapture animals to calculate proportion of actual population sample

  1. Capture animals, mark them, release, then recapture and count marked individuals

  2. Lincoln Index Formula:

  𝑁=(𝑛1×𝑛2)𝑚N=m(n1​×n2​)​

  • 𝑁N = Estimated total population size
  • 𝑛1n1​ = Number of individuals captured, marked, and released in the first sample
  • 𝑛2n2​ = Number of individuals captured in the second sample
  • 𝑚m = Number of marked individuals recaptured in the second sample
• Carrying capacity


  Maximum size of a population that an environment can support
• Competition for limited resources


  As population grows, more resources like water, oxygen, food are taken from the environment until a limit is reached
• Factors that limit population size

  • Food and water availability
  • Space for territories & nests
  • Availability of mates
  • Diseases
  • Predation
  • Environmental change
  • Build-up of toxic by products of metabolism
  • Injury
  • Senescence (death from age related illnesses)
• Density dependent factors


  Factors that control population size based on population density
• Density independent factors


  Factors that control population size regardless of population density
• As population grows


  Density dependent factors provide negative feedback to control population size
• Exponential growth curve


  Population grows exponentially until it reaches the carrying capacity
• Carrying capacity


  Competition for limited resources
• Resources animals and plants compete for

  • Food and water availability
  • Space for territories & nests
  • Availability of mates
  • Diseases
  • Predation
  • Environmental change
  • Build-up of toxic by products of metabolism
  • Injury
  • Senescence (death from age related illnesses)
• Density dependent factors


  Factors that depend on population size, e.g. competition, predation, food, disease, and parasitism
• Density independent factors


  Factors that affect all populations in a similar way, independent of population size, e.g. drought, wild fire, volcanic eruptions, hurricane, deforestation
• Population size can be limited by density dependent and density independent factors
• Density dependent factors


  • Competition
  • Predation
  • Disease, Parasitism, Infestation
• Density of a population

  Affects competition
• Density of a population

  Intensifies predation
• Density of a population

  Affects the spread of disease & pathogens
• Negative feedback control


  A type of self-regulating system, where any deviation from a steady-state is counteracted to promote stability
• Negative feedback control by density-dependent factors causes population size to fluctuate, resulting in it staying stable over time
• Population growth curves


  1. Exponential

  2. Transition

  3. Plateau
• Reproduction tends to cause exponential growth in populations as a result of positive feedback
• Density dependent factors lead to negative feedback that prevents exponential growth
• When any of the density dependent factors is absent, populations may grow exponentially
• Reasons for density dependent factors being absent

  • Extinction of predators, lack of competition, absence of diseases/parasites
• Example of population recovery


  • Bearded vulture in Switzerland
• Duckweed (Lemna sp.) is a good model organism for measuring sigmoidal population growth
• Modelling of the sigmoid population growth curve

  1. Place a small number of plants in a container

  2. Count the number of fronds (leaves) every day until the surface of the container is covered

  3. Plot your results – you should obtain a sigmoidal curve

  4. Consider different independent variables e.g. nutrient availability and the surface area of the container
• Community


  All of the interacting organisms in an ecosystem
• Intraspecific relationship


  Relationships between members of the same species
• Interspecific relationship


  Relationships between members of different species
• Categories of interspecific relationships

  • Herbivory
  • Predation
  • Competition
  • Mutualism
  • Parasitism
  • Pathogenicity
• Examples of interspecific relationships

  • Aphids feeding on plant sap
  • Ivy growing up oak barks
  • Potato blight fungus
  • Limpets feeding on algae
  • Anteater feeding on ants
  • Starfish eating oysters
  • Mycorrhizal fungi growing into roots
  • Bisons grazing on grass
  • Roundworm living in guts of racoons
  • Cheetah hunting gazelle
  • Tuberculosis infecting badgers
  • Clownfish living with sea anemonae
  • Birds picking teeth of crocodiles
  • Colibri feeding on nectar
  • Mosquito feeding blood
  • Cowbird laying egg in nest of Eastern phoebe
  • Tick feeding
• Mutualism


  An interspecific relationship that benefits both species
• Examples of mutualism


  • Root nodules in Fabacea
  • Mycorrhizae in Orchidaceae
  • Zooxanthellae in hard corals
• Predator-prey relationships


  Density-dependent control of animal populations